On the tabletop when you have dread ambush (lv3 ranger) you get 1 extra attack when you use your action to attack on your first turn, this means when you use action surge (lv2 fighter) to get another action to attack with you get another extra attack. Does this work the same on Baldur's Gate 3? I'm planning on making a 3 lvs in assasin rogue, 3 lvs in gloomstalker ranger and 2 lvs in fighter build.
To the best knowledge, all bonuses work independently, so you can't get a bonus triggering a bonus.
If you have a Dread Ambusher feat from the Gloom Stalker class:
On the first turn of combat:
Movement Speed increases by 3m.
Gain an additional attack that deals +1d8 damage.
and the Action surge from the lvl 2 fighter
Immediately gain an extra action to use this turn.
Assuming you have only one regular attack, you can:
- Get the bonus attack from Gloom Stalker, but only on the first round
- Use action surge and attack again.
BUT, saying all that, your build is very sub-optimal, as you won't get the "Extra attack" feat, which is available for
Barbarian at level 5
Fighter at level 5 and level 11
Monk at level 5
Paladin at level 5
Ranger at level 5
College of Valour at level 6
College of Swords at level 6
So you would do much better if you get either 5 levels of a ranger or 5 levels of a warrior. There is a similar build for Astarion here with 5 Gloom stalker, 4 Rogue (thief is better than assassin) and 3 Battle master fighter.
With the build above you can get up to 5 attacks in the first round (more with certain weapons and feats)